There are currently a large number of color display technologies, which are widely used in a printing technique such as a printer or the like, a display device, etc. These color display technologies are roughly classified into:                1. a method wherein gradation is reproduced through pseudo-halftone representation such as dithering by means a device capable of displaying a discontinuous gradation color, and        2. a method wherein halftone is reproduced by means of a device capable of displaying a substantially continuous gradation color.        
With respect to 2, it is possible to effect full-color display with no problem. For example, in a liquid crystal device using color filters of three colors of RGB, the respective display colors have analog gradation performance, so that it is possible to effect complete full-color display by spatial additive color mixture principle. Further, in a time-division color liquid crystal display method, light sources for three colors of RGB are switched at high speed and in synchronism therewith, a display device is subjected to among gradation control at high speed, so that it is possible to effect complete full-color display by timewise additive color mixture principle.
Even in the case where the display device itself has no gradation displayability, it has also been known that a substantially continuous gradation color can be displayed by effecting display at high speed through an ON/OFF operation. For example, plasma display panel (PDP) which has been widely popularized as a flat television, an organic EL display of the type wherein gradation display is effected by time division, a digital mirror device (DMD) which controls a display state by switching a mirror surface formed on a semiconductor substrate at high speed, and a method similarly using a ferroelectric liquid crystal (FLC), and the like are applicable.
As the color display technology other than the displays, there has been known a multi-value continuous gradation recording method using a density gradation method such as a laser intensity modulation in a laser-writing type printer or the like.
On the other hand, with respect to 1, it is put into practical use in the printer technology such as ink jet or laser beam, a bistable-type FLC display device, and the like. In these, a minimum display unit itself only has a discontinuous gradation displayability but pseudo-halftone display is effected by combining a plurality of display units and using the spatial additive color mixture effect.
This method includes one wherein the display medium itself can be controlled continuously but discontinuous gradation display is effected due to constraints of a control circuit. For example, a liquid crystal display device for effecting pseudo-full color display by using inexpensive driver ICs for 4 bit gradation in combination with dithering has been put into practical use.
Further, in the above described PDP, a phenomenon which is called pseudo-contour is visually recognized at the time of displaying motion picture in some cases. However, there is also a technique for obviating it by the spatial additive color mixture effect such as dithering. This is such a technique as to remedy a phenomenon that discontinuous gradation display is effected due to a visible factor at the time of displaying motion picture even in the case where a substantially continuous display is effected with respect to a still picture (image). In other words, there is also such a case where the above described case of 2 is applied to the still picture and the above described case of 1 is applied to the motion picture.
As described above, there are various display devices and color display is widely popularized but all the existent display methods are classified into the above described two methods. More specifically, the classified methods are only two methods consisting of a method of reproducing full-color display as it is by means of a device having an analog gradation displayability and a method of effecting pseudo-halftone display by using a device having a digital-like (discontinuous) gradation displayability in combination with a plurality of unit pixels and using the spatial additive color mixture effect.
On the other hand, e.g., in display of an interference color by an electrically controlled birefringence (ECB) effect in liquid crystal, in the case where an optical path difference in small, it is possible to effect continuous brightness modulation, and in the case where the optical path difference is larger than a predetermined value, the method is such a color display method that hue is changed while substantially retaining a brightness. In this case, only two display states of ON and OFF are present. More specifically, the method can be said to be a display mode in which analog gradation display and digital gradation display are co-present in a single pixel. This can be said to be a particular display method which is not applicable to any of the above described two methods.
As a method of providing multiple colors by using the color display based on the ECB (effect), a method wherein a plurality of pixels placed in different display states are combined has been disclosed in Japanese Patent No. 03098112. In this patent, a point that a unit pixel capable of effecting ECB color display is divided into two or more portions in an ECB-based color display device and different voltages are applied to the two or more portions to effect multi-color display has been disclosed.
Color display using the ECB effect has been little put into practical use. This is because a gradation displayability is inferior to those of other display methods. Although a proposal has been made by the above described patent document with respect to such a method that the unit pixel is divided and the respective ECB colors are combined to provide multiple colors, a gradation display method capable of providing a higher definition intermediary color has been required.